Since the dawn of semiconductor technology in 1950s, various studies have been conducted on a radiofrequency oscillation phenomenon (see, for example, Non-Patent Literature 1) and a breakdown phenomenon (see, for example, Non-Patent Literature 2) in a Si-based p-i-n diode. These phenomena in power devices made operable at an increasingly higher speed lead to malfunctions of peripheral circuits and surge breakdown of the devices. In recent years, these phenomena have again attracted attention (see, for example, Non-Patent Literature 3).
It is known that in a high-speed recovery diode these phenomena are noticeable under hard recovery conditions, such as a high Vcc, a high wiring inductance (Ls), a low operating temperature and a low current density (JA) (see, for example, Non-Patent Literatures 5, and 11). Solutions to the above-described problem with high-speed recovery diodes have been attained by enabling “soft recovery”, e.g., by adopting a thick n−-type drift layer or a thick n-type buffer layer and by applying a lifetime control technique (see, for example, Non-Patent Literatures 5 to 7). These methods, however, entail trade-offs among EMI (Electromagnetic Compatibility) noise, the breakdown tolerance and the total loss, and it is difficult to ensure a high level of compatibility among them.
On the other hand, the main characteristics of diodes have been remarkably improved by means of diodes having a p+-type layer in their back surfaces (see, for example, Non-Patent Literatures 4, 8, and 9), including RFC diodes (see, for example, Non-Patent Literatures 10 to 14). As further development problems, however, a challenge to extend the operating temperature range at the high-temperature side by reducing a leak current, a challenge to improve the maximum breaking current density by reducing VF (a voltage drop when the diode is turned on) in a high current density region, a challenge to improve the avalanche tolerance by strengthening the buffer structure are left.
A diode having an n-type buffer layer provided between an n−-type drift layer and an n-type cathode layer and having a medium impurity concentration between those of the n−-type drift layer and the n-type cathode layer has been proposed (see, for example, Patent Literatures 1 and 2). While no concrete numeric value of the concentration gradient in the n-type buffer layer is described in Patent Literature 1, a concentration gradient of 8×103 cm−4 can be estimated from FIG. 3 in Patent Literature 1. The n-type buffer layer in Patent Literature 2 is of the construction described in Non-Patent Literature 10 and the concentration gradient therein is 1×105 cm−4.